This invention relates generally to workpiece handling apparatus and, more particularly, to apparatus for handling workpieces for external grinding operations.
Handling apparatus are known for use in centerless grinding machines by which workpieces are automatically transferred or exchanged between a pickup position at holding means for unground workpieces and a load position at workpiece supporting means, e.g., sliding shoes, situated in operative relationship with a grinding wheel. Such conventional handling apparatus increase the rate of production of the grinding machine by reducing the time spent in loading and unloading the workpieces at the supporting means. More particularly, certain handling apparatus are known wherein an unground workpiece is transported from holding or feed means to the load position at the grinding wheel at the same time as a finished or ground workpiece is unloaded from the load position and transported to discharge means.
In this connection reference is made to U.S. Pat. No. 2,912,798 and German Pat. No. 1,954,487 which disclose apparatus for loading and unloading workpieces simultaneously in order to increase the rate of production of the machine. Such handling apparatus generally comprise pivotally mounted loading and unloading arms which carry a workpiece between loading and unloading positions.
However, conventional workpiece handling equipment of the type described above are definitely limited in that only a single workpiece can be ground at any one time and only a single workpiece can be loaded and unloaded subsequent to such grinding. Therefore, the rate of production of the grinding machine will not exceed a certain maximum value regardless of the speed at which the finished workpiece is unloaded from the load position and a new unground workpiece loaded in its place.
It is of course well recognized that the rate of production of a grinding machine can be increased by grinding two workpieces at the same time at respective staggered positions on the same grinding wheel. This technique however has not been fully utilized for the reason that although the actual grinding time per workpiece ground is reduced, the time required for transferring the unground workpieces to their respective load positions at the grinding wheel is increased. Thus, as noted above, conventional automatic handling apparatus of the type disclosed in the above-mentioned patents can only load a single unground workpiece and unload a single ground workpiece after the grinding operation. Therefore, it is seen that it is desirable to provide automatic workpiece handling apparatus whereby a pair of unground workpieces can be transported from holding or feed means to a respective pair of load positions at the grinding wheel and at the same time unload a pair of finished or ground workpieces from the load positions and transport the same to discharge means.
Problems arise in connection with the design of handling apparatus whereby a pair of unground workpieces are automatically transferred from a pair of pickup positions at holding means for unground workpieces to a pair of load positions at staggered locations on the circumference of a grinding wheel and wherein a pair of finished or ground workpieces are simultaneously transferred from the same load positions to discharge means for the finished workpieces. More particularly, it is generally necessary that the workpieces be ground in a rapid manner with precise accuracy. In order to obtain such speed and accuracy in a centerless grinding machine equipped with sliding shoes for supporting the workpiece, the shoes must be positioned with respect to each other at an optimal angular relationship and such that the workpiece is sufficiently enveloped thereby. However, since workpiece handling apparatus of the type described above, i.e., wherein the workpiece is held at the end of pivot arms which swing back and forth to perform the loading and unloading of the workpieces, require that a free path be maintained for the workpiece over the entire length of its motion while held on the end of the pivot arm, it has not been possible to obtain the optimal positioning of the sliding shoes where two workpieces are adapted to be loaded at staggered locations along the periphery of the circumference of the grinding wheel. Thus, the rear or upper sliding shoe would interfere with the path of a pair of workpieces being transferred to such staggered loading or chucking positions. Moreover, it would appear that the workpieces which could be handled would be limited in size in order to avoid mutual interference during transfer.